Three-Dimensional Response Characteristics of Transient Electromagnetic Method with Electrical Sources in Shallow Sea Areas Under Complex Geological Conditions
Compared with the frequency domain electromagnetic method,the transient electromagnetic method can effectively distinguish between air waves and seafloor electromagnetic response,which has a good application prospect in the shallow offshore oil and gas reservoir detection.The geological conditions of offshore area are generally more complicated,and there are strong cut terrain and complex structures,making the electromagnetic field response characteristics become extremely complex.These problems will bring great difficulties to the data interpretation work.In this paper,based on the time-domain finite element method(FEM),the unstructured mesh is used to dissect the complex geological model.By constructing the time-domain finite element equations and combining the long wire source approximation technique with dipole discretization and the backward Euler discretization technique,the three-dimensional forward modeling simulation of electromagnetic transient with electrical source in shallow sea area under complex geological conditions is realized.After verifying the accuracy of the algorithm,the influence of different seawater depths on air waves and the target body of seafloor oil and gas reservoir is analyzed through the three-dimensional forward modeling of the complex geological model.Based on these results,the influence of different surrounding rock resistivity and seafloor topography on the resolution of the target body of oil and gas reservoir is further analyzed.The results show that under shallow sea conditions,the pulse response is greatly affected by air waves,while the step response is less affected by air waves.As the depth increases,the influence of air waves decreases,and the resolution of oil and gas reservoirs also decreases.The electrical resistivity of surrounding rocks and the complex seafloor topopraphy have a serious impact on the transient electromagnetic properties of electrical sources.
transient electromagnetic methodshallow seaunstructured grid3D forward modelingtime domain finite element